1. Field of the Invention
This invention relates generally to output stages for bipolar monolithic integrated circuits, and more particularly, to rail-to-rail output stages that have constant product output characteristics.
2. Description of the Related Art
A typical class AB output stage for a conventional operational amplifier implemented with bipolar junction transistors (BJTs) is shown generally at 10 in FIG. 1. Output current is provided by transistors Q101 and Q102 which are connected "emitter-to-emitter". Transistor Q101 sources current I.sub.P, and transistor Q102 sinks current I.sub.N to provide an output current I.sub.L having symmetric drive characteristics. Diodes D101 and D102 form a translinear loop with Q101 and Q102 to create a small quiescent current through Q101 and Q102, thereby reducing crossover distortion that occurs when the output current swings through zero. Diodes D101 and D102, which are typically fabricated from diode-connected transistors in a monolithic implementation, are biased by the bias current I.sub.Q generated by transistor Q103.
The translinear loop formed by D101, D102, Q101 and Q102 imparts a constant product characteristic to the output stage. This loop can be analyzed using the translinear principle which provides that in a closed loop containing an even number of ideal junctions, arranged so that there arc an equal number of clockwise-facing and counter-clockwise-facing polarities, with no further voltage generators inside this loop, the product of the current densities in the clockwise direction is equal to the product of the current densities in the counter-clockwise direction.
The emitter areas of Q101 and Q102 are each Me where "e" is a unit emitter area. Thus, the current density in the base-emitter junction of Q101 is I.sub.P /Me, and the current density in the base-emitter junction of Q102 is I.sub.N /Me. The emitter areas of the transistors forming D101 and D102 are unit areas "e". Therefore, the current densities in the base-emitter junctions of D101 and D102 are each I.sub.Q /e. Applying the translinear principle to these current densities results in the following equation: EQU I.sub.N /Me.multidot.I.sub.P /Me=I.sub.Q /e.multidot.I.sub.Q /e(Eq. 1)
Rearranging terms provides the following result: EQU I.sub.N I.sub.P =M.sup.2 I.sub.Q.sup.2 (Eq. 2)
Thus, the product of I.sub.P and I.sub.N is equal to a scaling factor times the bias current squared.
A disadvantage of the circuit of FIG. 1 is that the voltage at the emitter of transistor Q101 must remain at least a diode voltage drop ("V.sub.BE ") plus a saturation voltage V.sub.CE for the current source transistor Q103 below the positive power supply rail VP, and the emitter of transistor Q102 must remain at least a V.sub.BE plus a saturation voltage V.sub.CE for the current source transistor Q104 above the negative power supply rail VN or else the transistors will turn off. Thus, the maximum voltage swing available at the output terminal VOUT is at least 2.times.(0.8+0.2) or 2.0 volts less than the total available power supply voltage (VP minus VN). In low power supply voltage circuits that operate with power supplies of 2.7 volts or less, this only allows an output voltage swing of less than 1 volt.
To improve the swing range of the output stage, amplifiers have been made having the output transistors connected "collector-to-collector" as shown in FIG. 2. This greatly improves the theoretical output swing because the collectors of output transistors Q.sub.P and Q.sub.N can operate to within about 0.2 volts of the positive and negative power supply rails V.sub.P and V.sub.N, respectively. Thus, the circuit of FIG. 2 is often referred to as a "rail-to-rail" output stage. However, the output transistors are more difficult to drive because the collector-to-collector connection of Q.sub.P and Q.sub.N prevents the use of the simple two diode biasing scheme used in the circuit of FIG. 1.
Circuit that utilize the general topology shown in FIG. 2 can generally be classified into two groups: (1) circuits that use a current mirror approach to driving Q.sub.P and Q.sub.N ; and (2) circuits that use some type of constant-product control (between the positive and negative portions of the output stage). Although circuits that provide accurate drive under ideal conditions have been devised, they arc generally complicated which causes them to perform poorly under many of the different practical conditions that are demanded of operational amplifiers, particularly large-signal transient conditions.